'Originally, in the late 19th century, jackpot was a term used in a form of poker, where the pot or pool accumulated until a player could open the betting with a pair of jacks or higher cards. It is now used of any large money prize that accumulates until it is won.'
...or some thing about jacket potatoes...

I was talking to someone about jacket potatoes, and she abbreviated to "jacket pot", and then I abbreviated to "jack pot", and then I was like "omgwtf JACKPOT!", so I thought that must be where "jackpot" came from because it made SO MUCH sense, and then I thought it must therefore come from Ireland because the Irish like potatoes lots and lots and lots, so FUCK YOU.